Where was Naomi‘s mother throughout the novel Obasan?

Obasan is a novel by Joy Kogawa that takes an intensely personal look at the lives of Japanese immigrants during and after the period of time when they were forcibly kept in internment camps in Canada (and the United States) during World War II.
Naomi Nakane is a Japanese Canadian woman who lived with her family in Vancouver, Canada when she was a young child. She and her family, as Japanese immigrants, were forced to leave their homes and live in an internment camp during WWII, and even for some time after the war was over. Naomi and her brother were raised by their Obasan (Japanese for "Aunt") and Uncle, because their father was separated from them for a long time and forced to go to a work camp, where he eventually died from tuberculosis. Naomi's mother went to Japan in 1941 to take care of a sick relative before her family was forced into internment camps and never came back.
As an adult, Naomi searches through her own past. Her family eventually decides to tell her the truth of why her mother never returned from Japan. Naomi's mother, Grandma Kato, and some extended family were in Nagasaki the day the city was bombed by the US Air Force in 1945. Naomi's mother and Grandma Kato survived the nuclear bombing, but her mother was horribly wounded and lost part of her face. In Grandma Kato's letter, she is described as "utterly disfigured." After the bombing, Naomi's mother spent a while recovering in a hospital where she "was expected to die, but she survived." Aunt Emily tells Naomi that "a missionary found [her] name on a plaque of the dead" on a Canadian maple tree. They do not know exactly when she died. Naomi's mother specifically told her relatives that she didn't want Naomi and her brother to know about what happened to her, and so she remained a mystery to her children up until this point.